1. Field of the Invention
The invention generally relates to a wafer support pedestal of a semiconductor wafer processing system. More particularly, the invention relates to an apparatus and a method for detecting the presence and position of a semiconductor wafer upon a wafer support pedestal.
2. Description of the Background Art
Semiconductor wafer processing equipment generally contains a vacuum chamber having a wafer support pedestal positioned within the vacuum chamber to support a wafer within the chamber atmosphere during processing. The pedestal generally contains a mechanism for retaining the wafer upon the pedestal surface. Such wafer retainers include mechanical clamps which physically retain the edge of the wafer and press the wafer against the pedestal surface, vacuum chucks that retain the wafer by establishing a vacuum beneath the wafer relative to the chamber pressure and electrostatic chucks that electrostatically retain the wafer in a stationary position upon the surface of the pedestal.
No matter what form of chuck is used to retain the wafer, it is important to determine when a semiconductor wafer has been positioned upon the pedestal and whether the wafer has been chucked (clamped) prior to processing the wafer. U.S. Pat. No. 5,436,790 issued Jul. 25, 1995 discloses a wafer presence and clamp condition monitoring apparatus. In this apparatus, a circuit monitors capacitance between two electrodes embedded within a wafer support pedestal. The capacitance falls into one range with no wafer positioned upon the support surface and into a second range with a wafer in place but not clamped. Furthermore, the capacitance falls in a third range with the wafer held in place by an electrostatic chuck formed when the embedded pair of electrodes are energized with a DC voltage. The monitoring circuit senses when the capacitance of the system is in each of the ranges by converting the measured capacitance to a DC voltage that can easily be sensed and used to confirm wafer placement and clamping.
Specifically, the electrostatic chuck used in the prior art system contains a pliable surface such that when the clamping force is applied, the wafer compresses the surface material and the wafer physically moves nearer to the pedestal surface and its embedded electrodes. This physical movement of the wafer relative to the electrodes causes a change in the capacitance between the electrodes. Such pliable surface materials are only useful in low temperature semiconductor processing systems. At high temperatures, these materials breakdown, outgas and/or deform. As such, at high temperatures, the electrostatic chuck having a pliable surface would contaminate the chamber.
Ceramic electrostatic chucks that are typically used in high temperature semiconductor wafer processing are constructed of a ceramic material that becomes somewhat conductive at high temperatures (i.e., the resistivity of the material decreases with increased temperature). Specifically, when the wafer rests flush against the surface of the chuck body while chucking voltage is applied one or more embedded electrodes, the wafer is primarily retained against the ceramic support by the Johnsen-Rahbek effect. One example of such a ceramic chuck is disclosed in U.S. Pat. No. 5,117,121 issued May 26, 1992 and incorporated herein by reference.
Ceramic chucks of this type have a hard, non-pliable surface that does not break down or deform during high temperature wafer processing. As such, the capacitive wafer position monitoring systems such as that described above have not heretofore been used in conjunction with these chucks because the surface is not pliable to allow the wafer to move substantially closer to the electrodes when chucked.
Therefore, a need exists in the art for a system that detects wafer presence and operates at high wafer processing temperatures.